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Human society is effectively gone and a new, intelligent species of Chimpanzees has taken over the role as the dominant species on Earth. It has been 40,000 years since the humans fell, so we can assume that all human technology has long been destroyed. Chimp archaeologists are now digging into the fossil records and finding anomalies that can’t be explained. These anomalies suggest that some past intelligent species existed on planet Earth, and that this species was very prolific. My question is: what strange, unexplainable inconsistencies would humans leave behind in the fossil records?

By inconsistent I mean stuff that would never occur naturally and must have been caused by the meddling of intelligent beings on nature.

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    $\begingroup$ This is an interesting question, but it's asking for an infinite list of things, which is off-topic. Can you help us understand what an "unexplainable inconsistency" is, and what criteria you will use to judge the best answer? $\endgroup$ – JBH Jul 3 '18 at 23:02
  • $\begingroup$ @JBH: Like stuff that wouldn’t occur naturally, and must have been caused by humans. $\endgroup$ – DT Cooper 2 Jul 3 '18 at 23:07
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    $\begingroup$ Do you mean like a spark plug? We need to narrow this down. Since any modern artifact will do, the real issue is a plausible explanation of how it survived 40K years and can't be excused as "our ancestors, 40,000 years ago." You should consider adding this to your question so we're not inviting an infinite list of things. $\endgroup$ – JBH Jul 4 '18 at 0:03
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    $\begingroup$ @a4android, I apologize, but do you need it to be literal? Every aspect of our technology and our biology is in play. Everything from a pair of scissors to a skull to the panama canal to any building, plant, animal, or piece of geology or geography that we've modified. Everything from carpet to the filling in a tooth to a domesticated cat. I apologize, but this question doesn't have a constrained list. I'm not a fan of closing questions. I haven't even yet voted to close this one. But there's no way for the OP to justify an answer as best. He's asked for an infinite list. $\endgroup$ – JBH Jul 4 '18 at 4:14
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It's pretty well agreed that Human's have left an indelible mark on the planet, enough that even a cursory inspection will reveal out presence for millennia to come. Quick list:

  • Mining Operations; the vast majority of the easily accessible minerals and resources have been extracted. Open pit mines have been dug, mountains have been leveled, oil reserves have been used
  • Cities; even some 40k years into the future, a buried city is going to be easily recognizable. Large concentrations of metals and pollutants, geometric landscaping
  • Landfills, especially with non-biodegradable plastics, will still be around.
  • Nuclear radiation; there are some test sites and zones that are going to be hot even so far into the future - nuclear waste storage dumps will also be easily recognizable
  • Climate Change; all that fossil fuel burning will leave easily recognizable trace elements all over the place - polar ice caps, ash layers etc etc
  • The Moon!; Bit further out - but the various pieces of tech should be undisturbed on the moon unless they get hit by an asteroid - no weather to wear them away
  • Mass extinction; even just going by the records of other animals, humans have been responsible for a mass die off of other species that should be easily recognizable to a future archaeologist.
  • Satellites; I'm not entirely sure about this one, but there still should be some (non-functional) debris and bits and bobs floating around in orbit

Essentially there's any number of things that a future race on Earth could use to discern our presence, even 40k years into the future, and that's without any special effort on our part. Once you factor in time capsules and "fling a light into the future" projects, it's pretty much certain

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    $\begingroup$ Re: mass extinctions - humans haven't been around for very long, in earth history terms, so future observers might not be able to distinguish between species we wiped out and species that were wiped out by whatever destroyed humanity. (Assuming that isn't itself obviously artificial - "hey, these craters are the wrong shape for meteors... and there are an awful lot of them, all in the same stratum...") $\endgroup$ – Cadence Jul 4 '18 at 6:56
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    $\begingroup$ Nuclear waste dumps will be recognisable in 40 000 years, yes. Not from radiation though, but from that they are their very durable facilities. Sweden's KBS-3 method calculates that a waste dump needs 100 000 years for the waste "to reach the same level of activity as the minerals it was once mined from". Fourty thousand years means you are just little over a half-life away from that. And an activity difference of about 2, is next to nothing. That gets lost in the noisy variation of ordinary background radiation. You need something with magnitudes of difference for it to be noticeable. $\endgroup$ – MichaelK Jul 4 '18 at 14:44
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    $\begingroup$ This Article is fairly relevant to the Cities and Landfills point. $\endgroup$ – Andon Jul 9 '18 at 2:54
  • $\begingroup$ as a note, most plastics will decompose within 1000 years (modern plastic shopping bags are an exception, as they don't biodegrade, but do photodegrade. If buried they may never decompose). Glass, however, is largely non-reactive, and will last for a long time. The sheer amount of glass that we've created will mean some glass will exist still and in forms that would be recognizable as non-natural. I imagine some chimpanzee finding a coke bottle. $\endgroup$ – Marshall Tigerus Jul 9 '18 at 15:40
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If we focus just on biologically originated fossils (as opposed to non-biological materials and artifacts), even there human civilization leaves a very prominent mark in history.

  1. Proliferation of new sub-species. Fossil records would indicate that in a matter of centuries, certain species have become totally dominant, and there is no natural explanation for it. Chickens, cows, pigs, sheep and goats have outstripped most other lifeforms, while undergoing quite abrupt evolutionary change along the way. Human fossil records also will be very prominent;
  2. Extinction of many older species. While extinctions happen naturally, this one would be going alongside the proliferation of domesticated animals and would not be seen as natural;
  3. Unusual geographic extension of fossil records. Animal bones and tropical fruit seeds will be found in all climate zones and all latitudes.
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If you're talking about 40,000 years in the future, probably another big one is that the carbon cycle will still be out of whack. It'll take longer than 40,000 years for all of the CO2 we've pumped into the atmosphere-ocean system to be re-absorbed into sediments.

Plastics are likely to persist since there are no organisms capable of digesting them. The only way to remove them at the moment is photooxidation (which causes incomplete degradation) and burning. Some metals will oxidize and be remobilized, but a lot of them will still hang around in the shape of what we created, if not in a reduced oxidation state.

Have you seen Gavin Schmidt's piece on this same idea, only with a longer timescale (55 million years)? https://motherboard.vice.com/en_us/article/3kj4y8/gavin-schmidt-fiction-under-the-sun

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  • $\begingroup$ Welcome to WorldBuilding nielsokr! Interesting answer. A little thing about your link though: links can get outdated, which would leave the last paragraph of your answer basically meaningless for future readers. The idea behind StackExchange is to have a repository of questions and answers for future readers. Please always try to summarize the most important parts of linked sites and only use links as a sort of "Here is a more detailed version of what I just sayed". If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Jul 4 '18 at 7:24
  • $\begingroup$ Actually, the half-life of CO2 in the atmosphere is about 40 years. This is long by human scales, but a "mere" thousand years will completely eliminate the human-generated CO2. $\endgroup$ – Mark Olson Jul 4 '18 at 14:45
  • $\begingroup$ In preserving the Apollo mission artifacts for museums, the problem they are having is that after 50 years in storage the plastics are deteriorating, while the natural materials (cotton canvas) is still good. $\endgroup$ – pojo-guy Jul 4 '18 at 14:58
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All of these assumptions assume that Humans just up and died. a gradual decline or massive war, or impact event could easily change some of these results.

Biological

While the large number of extinctions have already been covered in the other answers. its worth noting that humans bones would survive in certain places but definitely not all. the majority of climates in the world will be unlikely to provide reliable sources of human remains. however some places will, areas with very low humidity is a very good start. and those remains may have some form of prosthetic or obviously refined additions. Certain types of fillings will still be evident. along with metal plates holding bones together etc.

The problem is will they really survive... for most of the world no. the reason: it is becoming quite common to cremate our dead. far more common than it has been for a 100 years or so. so there will be evidence, but alas it will be hard to find

Non-Biological

Even "Non-Biodegradable" plastics, degrade over time, and 40,000 years is long enough for this to happen, they won't be gone completely but they would only appear as particulates in a sedimentary layer in Core samples, so anything made of plastic will most likely be gone. metal structures around the world would have collapsed and while it would be evident that some of this structures existed at some point the materials would be largely unclear at first sight (yes in depth analysis would show they were RSJs etc). but this wouldn't be as important as Steel. Steel does not occur naturally, it an Alloy, and the Modern Human World is basically made out of Steel. from buildings to car parts etc. this would be the first clue as to at least an industrial level society had lived here.

The Pyramids would still exists, yes it would continue to crumble, and most likely appear to be massive sand dunes, but once excavation happened on these big mounds they'd find the stone work to be definitively refined and cut, even to this day thousands of years after being built after the limestone has eroded from the faces and the stone beneath has begun to crumble. the stones that line the internal chambers are still in almost perfect condition.

Gold, nowhere in the world does gold occur naturally in such large and pure amounts than in National Reserve Stores. the Bank of England, US Federal Reserve etc all have large amounts of Gold stored away. if the future society found these, they'd be 100% certain that this was some form of financial store. even if everything surrounding the Gold had collapsed and turned to mud (not literally) the Gold would still be perfect evidence.

Any material in significantly higher purity than occurs naturally would point to a refining process. then each of those materials would then show what level that society was at: little Steel, little Gold, but high concentrations of Brass and Iron, pre-medieval times, Steel, Gold (not super high purity though) Brass Copper etc. points to medieval. Steel and gold, and copper etc, industrial. and then Palladium super pure Aluminium, pure gold etc. Modern age.

Radioactive

if we assume the future society uses anything similar to Carbon Dating, then they will quickly discover that there was a huge amount of radioactive material in the atmosphere at one point. Carbon/lead dating easily show these effects from the atomic testing in the 40s and 50s to the Chernobyl Disaster and presumably Fukushima as well. and again certain materials point toward nuclear tests. Cesium isotopes for example are evident in lake beds world wide after nuclear tests and these show up in Lead Dating techniques

Space

While earths Satelites will have all re-entered in 40,000 years, the third stage of Apollo 12, is in incredible high orbit of earth, it didn't reach its intended trajectory and got flung out into solar orbit and then re-entered Earths Sphere of influence, at this time it is unclear if this will enter Earths Atmosphere but its possible it'll be out there for good.

The Lunar landing sites are have test equipment and parts that will degrade over time due to solar radiation, but most of it will take so long that they will survive 40,000 years

Voyager 2 is on its way out of the solar system, it was nothing really to worry about, it will probably be pretty banged up in 40,000 years, but it will still be flying out into space for someone to find.

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After 40,000 years there won't be much left of most of our cities and our mines and landfills will only be tiny geographic samples that are easy to miss in a widespread survey. There are a few things that will survive that long though.

  • Processed Gold and Platinum jewelry, they'll be widespread, found in the forests that grow over our graveyards and in the gravelly alkaline soils where our concrete cities have crumbled and they won't corrode they'll be there a long time.

  • Other Gold artifacts like bullion bars or coins will similarly last a long time and show clear evidence of written symbols etc...

  • Cut Gems in general will be a sign of artificial tampering to someone who knows what they're looking at.

  • Mount Rushmore and similar monuments and smaller statuary in highly stable rocks like Granite or better yet Quartzite. These will last millions or even billions of years in a recognisable form depending on how geologically stable the area is.

  • Radiation, there will be pockets of intense radiation from nuclear power plants, and nuclear waste storage facilities. There will also probably be small low level but detectable traces from every hospital and clinic that once did radiotherapy.

  • Monumental structures, the Egyptian Pyramids, the Great Wall of China, Uxmal and it's peers in the Americas, Stone Hedge, anything that's really big and made of stone will be eroded but still there.

  • Isotopic traces, there are a number of longlife isotopes like Uranium-236 that don't normally exist in nature and have half-lives measured in millions of years that we've produced very large quantities of that will still be around.

  • Chemical traces, as many of our artificial polymers decay they produce organic (as in based on carbon chains) compounds that don't occur in nature and so will persist in water, soil, and wildlife because they don't take part in the carbon cycle until broken down further by UV Radiation.

  • Cutlery and DeLoreans, and anything else made largely of Stainless Steel, Stainless Steel will remain in whatever form it was cast in almost forever unless exposed to a highly acidic environment or extreme heat.

  • Elemental Aluminium, in nature Aluminium is only ever found as $Al_2O_3$ at various levels of purity, so to find even small amounts of it in it's pure reduced form would be quite telling. Because of the structure of the Aluminium Oxide that forms on the pure metal it prevents further corrosion in most environments so things like Aluminium engine blocks are still going to be large unaltered after only 40,000 years in salt-free conditions.

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    $\begingroup$ Isaac Asimov wrote a short story in which certain porcelain items were the only thing far future archaeologists found on Earth. Each house had one, so they concluded they were ceremonial objects... $\endgroup$ – Mark Olson Jul 4 '18 at 14:48
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LDPE plastic can last 40,000 years in cold underground conditions. it's mostly susceptible to UV's and light, so that's toys, bottle tops, tools, glasses, plastic bags.

Mountain road works, mines and tunnels cut into rock can last 1-5 million years. We can find Neanderthals and their flint tools in great numbers from short time spans around 40k years ago, on riverbanks and uncovered by wind in the Sahara.

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Based on thousands of road and rail works that will stay visible for 1-10 million years, they can dig to investigate them. They will find road, litter, barriers, and the road will lead to cities. So when you uncover a human road in the next 5 million years, a bit of radar detective work may reveal all the towns in the country.

a 12000 year old temple, Göbekli Tepe was only 2-10 meters underground, so a city like Dubai and Dallas would still be partially uncovered by erosion for 100,000 years, exposing bands of tarmac (which stays recognizable for millions of years), millions of bottles, metal, cars made of zinc and aluminium, chunks of rust the size of boats and cars. I'd give some major cities 20 to 50 million years, to cover the last cement bridge pillars and durable monuments.

Humanity's fossil imprint is easily 1000 times more massive than a neanderthal's, in kilotons of artificial constructs, and many times more durable, so you'd have to extend the time frame to about 500,000 years or 20 million years to make human's imprint difficult to find. A chimp would take about 2-10 million years to evolve to a human sophistication level.

Common and clear signs of humanity will be easy to find for the next 1-5 million years. If humanity survives until 2500, the durability and quantity of the fossils may be about 10 times higher, so perhaps 10-20 mn years.

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